The present invention relates to feedback amplifiers, and more particularly to a multifunction amplifier having a finite transconductance to provide a finite output impedance.
A typical feedback amplifier has an open loop gain block with arbitrarily large gain. It has an input resistor and a feedback resistor between the input and output of the amplifier. The overall voltage gain depends only upon the input and feedback resistor values. The output impedance approaches zero ohms due to the arbitrarily large open loop gain. An amplifier which is intended to drive a transmission line needs to have an output impedance which matches the line impedance. To establish this finite output impedance, common practice is to provide a "buildout" resistor between the amplifiers zero impedance output and the line. The required output voltage swing at the output of the amplifier must be twice that at the input of the transmission line. Amplifiers of this type have been used, for example, as video amplifiers, such as in the Horizon Video Routing Switcher manufactured by Grass Valley Group, Inc. of Grass Valley, Calif.
Finite output impedance has also been achieved by the combined use of positive and negative feedback in audio output amplifiers, such as in the 3280 Digital Audio Multiplex System manufactured by Grass Valley Group, Inc. of Grass Valley, Calif. To achieve a specific output impedance a small value, less than the actual desired output impedance, buildout resistor is inserted in series with the output of the amplifier. One of the feedback paths is taken from the output of the buildout resistor to the positive input of the amplifier and the other feedback path is taken conventionally from the output of the amplifier to the negative input of the amplifier. The positive feedback raises the effective impedance of the small valued buildout resistor and reduces the required voltage swing at the output of the amplifier compared to that required with the conventional buildout approach. This in turn reduces the power supply voltage requirements.
For communications over a telecommunications network which uses a ternary coded digital signal, binary input signals have to be combined, shaped and buffered before being amplified for transmission. What is desired is a multifunction amplifier which uses minimal power supply voltage while accepting binary inputs and producing a ternary output suitable for transmission over the telecommunications network.